go/types: len(((*T)(nil)).X) is const if X is an array

Fixes #4744.

R=adonovan
CC=golang-dev
https://golang.org/cl/7305080

src/pkg/exp/gotype/gotype_test.go

@@ -181,7 +181,7 @@ var tests = []string{
 
 	// "runtime",
 	"runtime/cgo",
-	// "runtime/debug", // rejects a valid constant - issue 4744
+	"runtime/debug",
 	"runtime/pprof",
 
 	"sort",

src/pkg/go/types/builtins.go

@@ -72,7 +72,7 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, bin *builtin, iota
 	case _Cap, _Len:
 		mode := invalid
 		var val interface{}
-		switch typ := implicitDeref(underlying(x.typ)).(type) {
+		switch typ := implicitArrayDeref(underlying(x.typ)).(type) {
 		case *Basic:
 			if isString(typ) && id == _Len {
 				if x.mode == constant {
@@ -85,7 +85,11 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, bin *builtin, iota
 
 		case *Array:
 			mode = value
-			if !containsCallsOrReceives(arg0) {
+			// spec: "The expressions len(s) and cap(s) are constants
+			// if the type of s is an array or pointer to an array and
+			// the expression s does not contain channel receives or
+			// function calls; in this case s is not evaluated."
+			if !check.containsCallsOrReceives(arg0) {
 				mode = constant
 				val = typ.Len
 			}
@@ -382,10 +386,10 @@ Error:
 	x.expr = call
 }
 
-// implicitDeref returns A if typ is of the form *A and A is an array;
+// implicitArrayDeref returns A if typ is of the form *A and A is an array;
 // otherwise it returns typ.
 //
-func implicitDeref(typ Type) Type {
+func implicitArrayDeref(typ Type) Type {
 	if p, ok := typ.(*Pointer); ok {
 		if a, ok := underlying(p.Base).(*Array); ok {
 			return a
@@ -394,25 +398,25 @@ func implicitDeref(typ Type) Type {
 	return typ
 }
 
-// containsCallsOrReceives returns true if the expression x contains
-// function calls or channel receives; it returns false otherwise.
+// containsCallsOrReceives reports if x contains function calls or channel receives.
+// Expects that x was type-checked already.
 //
-func containsCallsOrReceives(x ast.Expr) bool {
-	res := false
+func (check *checker) containsCallsOrReceives(x ast.Expr) (found bool) {
 	ast.Inspect(x, func(x ast.Node) bool {
 		switch x := x.(type) {
 		case *ast.CallExpr:
-			res = true
-			return false
+			// calls and conversions look the same
+			if !check.conversions[x] {
+				found = true
+			}
 		case *ast.UnaryExpr:
 			if x.Op == token.ARROW {
-				res = true
-				return false
+				found = true
 			}
 		}
-		return true
+		return !found // no need to continue if found
 	})
-	return res
+	return
 }
 
 // unparen removes any parentheses surrounding an expression and returns

src/pkg/go/types/check.go

@@ -27,6 +27,7 @@ type checker struct {
 	objects     map[*ast.Object]Object            // maps *ast.Objects to their unique object
 	initspecs   map[*ast.ValueSpec]*ast.ValueSpec // "inherited" type and initialization expressions for constant declarations
 	methods     map[*TypeName]*Scope              // maps type names to associated methods
+	conversions map[*ast.CallExpr]bool            // set of type-checked conversions (to distinguish from calls)
 	funclist    []function                        // list of functions/methods with correct signatures and non-empty bodies
 	funcsig     *Signature                        // signature of currently typechecked function
 	pos         []token.Pos                       // stack of expr positions; debugging support, used if trace is set
@@ -399,6 +400,7 @@ func check(ctxt *Context, fset *token.FileSet, files []*ast.File) (pkg *Package,
 		objects:     make(map[*ast.Object]Object),
 		initspecs:   make(map[*ast.ValueSpec]*ast.ValueSpec),
 		methods:     make(map[*TypeName]*Scope),
+		conversions: make(map[*ast.CallExpr]bool),
 	}
 
 	// handle panics

src/pkg/go/types/conversions.go

@@ -40,6 +40,7 @@ func (check *checker) conversion(x *operand, conv *ast.CallExpr, typ Type, iota
 		x.mode = value
 	}
 
+	check.conversions[conv] = true // for cap/len checking
 	x.expr = conv
 	x.typ = typ
 	return

src/pkg/go/types/testdata/builtins.src

@@ -33,6 +33,10 @@ func _cap() {
 	assert(_4 == 20)
 	_5 := cap(c)
 	cap /* ERROR "not used" */ (c)
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = cap(((*T)(nil)).a)
 }
 
 func _close() {
@@ -151,6 +155,10 @@ func _len() {
 	var ch <-chan int
 	const nn = len /* ERROR "not constant" */ (hash[<-ch][len(t)])
 	_ = nn // TODO(gri) remove this once unused constants get type-checked
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = len(((*T)(nil)).a)
 }
 
 func _make() {